Solar Drinking Bird

The Drinking Bird is a classic science toy that dips its head up and down into a glass of water. It’s filled with a liquid called methylene chloride, and the head is covered with red felt that gets wet when it drinks. But how does it work? Is it perpetual motion?

Let’s take a look at what’s going on with the bird, why it works, and how we’re going to modify it so it can run on its own without using any water at all!

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The bird needs a temperature difference between the head and tail. Since water needs heat in order to evaporate, the head cools as the water evaporates.  This temperature decrease lowers the pressure inside the head, pushing liquid up the inner tube. With more liquid (weight in the head), the bird tips over. The bird wets its own head to start this cycle again.

The trick to making this work is that when the bird is tipped over, the vapor from the bottom moves up the tube to equalize the pressure in both sides, or he’d stay put with his head in the cup.  Sadly, this isn’t perpetual motion because as soon as you take away the water, the cycle stops. It also stops if you enclose the bird in a jar so water can longer evaporate after awhile. Do you think this bird can work in a rainstorm? In Antarctica?

What’s so special about the liquid? Methylene chloride is made of carbon, hydrogen, and chlorine atoms. It’s barely liquid at room temperature, having a boiling point of 103.5° F, so it evaporates quite easily. It does have a high vapor pressure (6.7 psi), meaning that the molecules on the liquid surface leave (evaporate) and raise the pressure until the amount of molecules evaporating is equal to the amount being shoved back in the liquid (condensed) by its own pressure. (For comparison, water’s vapor pressure is only 0.4 psi).

Note that the vapor pressure will change with temperature changes. The vapor pressure goes up when the temperature goes up. Since the wet head is cooler than the tail, the vapor pressure at the top is less than at the bottom, which pushes the liquid up the tube.

It really does matter whether the bird is operating in Arizona or the Amazon.  The bird will dip more times per minute in a desert than a rain forest!

Let’s find out how to modify the bird so it’s entirely solar-powered… meaning that you don’t have to remember to keep the cup filled with water.  Here’s what you need:

• drinking bird
• silver or white spray paint
• black spray paint
• razor
• mug of hot water
• sunlight or incandescent light

 
Download Student Worksheet & Exercises

In this modification, you completely eliminated the water and converted the bird to solar, using the heat of the sun to power the bird. Now your bird bobs as long as you have sunlight!

How does that work? Since the bottom of the bird is now black, and black absorbs more energy and heats up the tail of the bird. Since the tail section is warmer, the pressure goes up and the liquid gets pushed up the tube. By covering the head with white (or silver) paint, you are reflecting most of the energy so it remains cool. Remember that white surfaces act like mirrors to IR light (which is what heat energy is).

Questions to Ask: Does it work better with hot or cold water? Does it work in an enclosed space, such as an inverted aquarium? On a rainy day or dry? In the fridge or heating pad?

Exercises Answer the questions below:

1. Where does most of the energy on earth come from?
   1. Underground
   2. The sun
   3. The oceans
2. What is one way that we use energy from the sun?
3. What is the process by which the liquid is being heated inside the bird?
   1. Precipitation
   2. Pressure
   3. Evaporation
   4. Transpiration

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